Electrochemical enrichment of a community of weak electricigens and characterisation of three halotolerant electroactive isolates: Micrococcus sp. YH-1, Gordonia sp. RH-1 and Stutzerimonas sp. CH-1

Abstract

Microbial electroactivity in elevated salt conditions is seldom studied, despite both fundamental and applied relevance. This work reports the enrichment of electroactive microorganisms under saline conditions (2% NaCl), performed on screen-printed carbon electrodes with applied anodic potential. From the weakly electroactive enriched community, three halotolerant electroactive isolates were obtained in pure culture and identified via 16S rRNA gene sequencing. Two isolates were Gram positive (proposed names Micrococcus sp. YH-1 and Gordonia sp. RH-1) and one was Gram negative (proposed name Stutzerimonas sp. CH-1). The isolates were electrochemically characterised on conventional carbon felt electrodes using chronoamperometry. After two days of growth, a current density of 8.29 ± 0.85 μA/cm2 was observed in Micrococcus sp. YH-1, comparable to reported values for the model electroactive microorganism Shewanella oneidensis. Gordonia sp. RH-1 and Stutzerimonas sp. CH-1 appeared to be weak electricigens due to their low current output (2.19 ± 0.46 μA/cm2 and 1.73 ± 0.47 μA/cm2, respectively). The isolates are notable as there are very limited reports of electroactivity in the genera Micrococcus and Gordonia. Cyclic voltammetry revealed prominent redox peaks in Micrococcus sp. YH-1 and Stutzerimonas sp. CH-1. Scanning electron microscopy demonstrated colonisation of the electrode by each isolate, along with thin cellular appendages in Micrococcus sp. YH-1 and Stutzerimonas sp. CH-1. This work extends the catalogue of characterised halotolerant electroactive microbes while also enhancing our understanding of weak electricigens, extracellular electron transfer in Gram positives, and the inherent electroactivity of the natural environment.